Particulate, absorbent, polymeric compositions are capable of absorbing large quantities of fluids such as water and body exudates and which are further capable of retaining such absorbed fluids under moderate pressures. These absorption characteristics of such polymeric compositions make them especially useful for incorporation into absorbent articles such as diapers. For example, U.S. Pat. No. 3,699,103 issued to Harper et al. on Jun. 13, 1972 and U.S. Pat. No. 3,670,731 issued to Harmon on Jun. 20, 1972, both disclose the use of particulate, absorbent, polymeric compositions (also referred to as hydrogel, hydrocolloid, or superabsorbent materials) in absorbent articles.
Conventional particulate, absorbent, polymeric compositions, however, have the limitation that their rate of fluid uptake is much lower than that of conventional cellulosic fiber webs due to the small surface area to mass ratio of the constituent particles of the polymeric composition. The surface area to mass ratio of the particles of the particulate, absorbent, polymeric composition is important since it can control the overall fluid uptake rate of the bulk polymeric composition. The surface area to mass ratio, and hence the fluid uptake rate, can be substantially increased by decreasing the mass average particle size of the particles in the bulk polymeric composition. However, when these small particles or fines swell upon contact with liquids, the particles, when incorporated in a fiber web, tend to be easily forced into the interfiber capillaries of the web. The swollen or partially swollen fines may also form a mass of coagulated gel held together by fluid surface tension forces, thus forming a gel barrier. In either case, resistance to fluid flow through the structure is increased as fluid flow channels are blocked within the fiber web or the gel mass resulting in a marked decrease in permeability. These phenomena are commonly referred to as "gel blocking."
One attempt to break this trade-off between the rate of fluid uptake and gel blocking has been to agglomerate, via water, a multiplicity of small particles to larger "nuclei" particles. Such water-agglomerating techniques are disclosed in Japanese Patent Laid-Open SHO 61(1986)-97,333 and Japanese Patent Laid-Open SHO 61(1986)-101,586. While water-agglomeration of the particles does result in a modest increase in the fluid uptake rate due to an increased surface area to mass ratio of the larger particles, the water-agglomerated particles dissociate upon contact and/or swelling with an aqueous solution. This results in a concentration of swollen or partially swollen free fine particles that will contribute to an increased gel blocking effect via the mechanisms described above.
Another attempted solution to the problem has been to surface treat the discrete particles. One specific surface treatment method is to surface crosslink the discrete particles so that each individual particle has a higher crosslink density among the polymer chains at or near the surface of the particles. Such surface crosslinking techniques are described in U.S. Pat. No. 4,666,983 issued to Tsubakimoto et al. on May 19, 1987; and U.S. Pat. No. 4,734,478 issued to Tsubakimoto et al. on Mar. 29, 1988. Surface crosslinking of the particles results in a modest reduction in one form of the above-defined gel blocking by reducing the tendency of the discrete particles to coagulate into an impermeable gel mass during swelling. However, the rate of fluid uptake of the particles is not increased because the surface area to mass ratio of the particles remains relatively constant.
Therefore, the present invention seeks to resolve the above problems by providing improved particulate, absorbent, polymeric compositions having a high rate of fluid uptake with minimal gel blocking properties.
Thus, it is an object of the present invention to provide particulate, absorbent, polymeric compositions with a high rate of fluid uptake.
It is a further object of the present invention to provide particulate, absorbent, polymeric compositions that display minimal gel blocking properties.
It is a still further object of the present invention to provide particulate, absorbent, polymeric compositions that have a high resistance to compression during use (ie., during swelling) in order to maintain and/or increase the permeability of absorbent products incorporating such polymeric compositions.
It is an even further object of the present invention to provide particulate, absorbent, polymeric compositions having minimal dissociation of fine particles upon fluid contact or swelling.
It is an even still further object of the present invention to provide particulate, absorbent, polymeric compositions having minimal free fines in the dry state.
It is an even further object of the present invention to provide particulate, absorbent, polymeric compositions that achieve predefined rates of fluid uptake by selecting specific characteristics of the precursor particles such as the mass average particle size or absorptive capacity.
It is another object of the present invention to provide a method for producing such particulate, absorbent, polymeric compositions.
It is a further object of the present invention to provide improved absorbent products, absorbent members, and absorbent articles (such as diapers or sanitary napkins) incorporating the particulate, absorbent, polymeric compositions of the present invention.